Laser　welding　with　high　beam　intensity　generates　a　metal　vapor,　which　partially　displaces　the　metal　melt,　forming　a　narrow　keyhole-shaped　fusion　zone.　This　study　conducted　theoretical　analysis　of　the　keyhole　formation　processes　within　fiber　laser　welding,　based　on　three　types　of　experimental　observation.　During　the　welding　mode　transition　process,　the　keyhole　formation　was　abrupt　due　to　the　jump-like　absorptivity　variation　in　the　concave　molten　pool.　In　the　primary　stage　of　the　laser　keyhole　welding　process　(when　the　laser　was　turned　on),　the　keyhole　depth　grew　rapidly　and　then　slowed　down　before　reaching　a　steady　state.　The　keyhole　formation　time　was　related　to　the　welding　characteristic　time　(WCT),　determined　as　the　ratio　of　laser　spot　diameter　to　the　welding　speed.　In　the　steady　stage　of　fiber　laser　keyhole　welding,　the　keyhole　formation　process　was　a　periodic　motion　of　the　laser　beam　on　the　front　keyhole　wall　(FKW).　The　periodic　drilling　occurred　because　the　laser　beam　was　irradiated　on　the　FKW,　with　the　inclination　angle　fluctuating　around　the　Brewster　angle.